The present invention relates to a boring bar and a removable cutting insert having a plurality of edges which are indexed sequentially so that a new edge is available for cutting as an old edge becomes worn. The boring bar is particularly useful for machining rotary-type, internal combustion engines which have a metal alloy layer on the inner surface wall of the cylinder. The material is generally of a type which is called "transplant" material. Transplant material is very difficult to machine; and, moreover, the aluminum wall and ribs that lie behind the transplant material vary in stiffness around the epitrochoidal surface of the cylinder block. Thus, a substantial increase in the normal cutting force of the machining tool for the transplant material can deform the housing unevenly resulting in form error.
The transplant material on the cylinder walls of a rotary engine block might be ground or milled. However, these machine processes have decided disadvantages which make them unattractive. Grinding is very time consuming because of the amount of transplant material that must normally be removed. In addition, in grinding, special precautions must be taken to compensate for the deflection of the housing at various sections during the grinding operation. Milling also takes too long of a time to be efficient; and, in addition, milling will generally not be as accurate as boring due to cutter deflection and tool wear. Milling cutters also will not normally stand up long enough to give a satisfactory finish and size at an economical rate. In addition, milling machines generally will be more complicated and expensive than a boring machine which utilizes the boring tool and insert of the present invention.
It is emphasized that the boring tool and insert of the present invention are useful for general purpose boring applications. However, the tool is especially useful in forming an epitrochoidal surface, as previously mentioned. In particular, it may be employed in the boring machine disclosed in U.S. Pat. No. 3,812,746 issued May 28, 1974, in the name of Oliver Edward Saari, and assigned to Illinois Tool Works Inc. The machine of the Saari patent was designed to rapidly form an epitrochoidal surface within a recessed cylinder bar with a high degree of accuracy but without vibrations which might otherwise result in unbalanced machine operation. This machine was designed so as to maintain the rapidly moving parts of the machine and the relatively heavy cylinder bar in kinetic balance. A single tool element or cutter, such as a conventional boring bar, was disclosed in the prior Saari patent.
In the machine of the Saari patent, a unique mechanism was provided for coupling the rotary tool actuating device or spindle and the boring bar, which was slightly offset from the spindle axis. This mechanism was operable so as to precisely locate the cutting extremity of the supported tool in contact with the surface that defines the cavity being machined in the work piece so that as the work piece rotated at a relatively high speed about a fixed axis, coincident with the center of the cavity, an accurate epitrochoidal surface was formed. The coupling means between the tool actuating means and the tool supporting means was oscillated within predetermined limits about the fixed axis, thereby maintaining the fixed axis and the point of engagement of the tool with the surface defining the cavity in a common plane extending normal to a line tangent to the epitrochoidal surface at the point of contact between the tool and the work piece.
Cutting tools, such as those found in conventional boring bars, are expected to last for the machining of many parts before the tool has to be adjusted to hold the part size, or before the insert is indexed to a new cutting edge. The mechanism of the present invention, on the other hand, utilizes a relatively inexpensive removable cutting insert, which is indexed to a new point after every cycle of operation during the time period, while a finished housing is being unloaded and a new housing is being loaded on the boring machine. Each insert is manufactured accurately so that once the tool is positioned and established, the tool is not repositioned when the insert is indexed or when it is replaced.
Only one or two work pieces, such as rotary cylinder blocks, will be bored by each cutting edge, but there are a large number of cutting edges per insert on the tool; and a relatively large number of cylinder blocks may be machined without replacing the insert. The insert cutting edges are designed to optimize the surface finish with rapid-feed, minimal normal cutting force and minimal dulling of the cutting edge during the boring operation. The cutting tool is indexed automatically in the boring bar during each cycle of operation, and it is removable from the boring bar; and a new insert is insertable in a particularly simple and fast manner.
It is an object of the present invention to provide an improved boring bar and removable and indexable cutting insert which may be employed in the machine of Saari patent and in other boring machines.